VOL. LVir.] PHILOSOPHICAL TRANSACTIONS. 425 



or 6th, and this may very possibly be the case, we must then increase the dis- 

 tances computed above, in the proportion of 2 or 24- to 1, to make the sun's 

 light no more than equal to theirs; and his parallax would be less, in the same 

 proportion, than those already mentioned.* 



On the supposition then, that the fixed stars are of the same magnitude and 

 brightness with the sun, it is no wonder that their parallax should have hitherto 

 escaped observation ; since, if this is the case, it could hardly amount to 2 se- 

 conds, and probably not more than 1 in Sirius himself; though he had been 

 placed in the pole-|- of the ecliptic, and in those that appear much less luminous, 

 such for example as y draconis, which is only of the 3d magnitude it could 

 hardly be expected to be sensible with such instruments as have hitherto been 

 used in search of it. 



We have assumed the magnitude of the fixed stars, as well as their brightness, 

 to be equal to those of the sun; it is however probable that there may be a 

 very great difference among them, in both these respects ; and how much soever 

 we may therefore be wide of the truth, in attempting to fix the distance of par- 

 ticular stars from this reasoning, yet there is a very great probability that their 

 mean distances, settled by this method, will not be much out, some exceeding 

 and some falling short of it. And perhaps the consideration that a star must be 

 1000 times as great, cgeteris paribus, to appear equally bright, if it is placed at 

 10 times the distance, may serve to make it probable that the limits of the errors, 

 which we are likely to commit, in judging by such a rule, are not so great as 

 we might otherwise imagine them to be. 



With regard to the difference there may be in the native brightness of dif- 

 ferent stars, though it is probably very considerable, yet we can hardly suppose 

 that it is equal to their difference in magnitude, at least if we except those 

 which are subject to certain changes, and which for that reason we may suppose 

 to be luminous in some parts of their surfaces only. In other instances we may 

 perhaps judge in some degree of the native brightness of different stars with res- 

 pect to one another by their colour, those which afford the whitest light, being 

 probably the most luminous. J 



• The light which we receive from the full moon, according to M. Bouguer's experiments, in the 

 work above-mentioned, is only a 300000th part of that which we receive from the sun, whereas it 

 ought to amount to no less than a 45000th part of it, according to the principles above made use of 

 in computing the quantity of light derived from Jupiter and Saturn) so that the moon, as appears 

 fram these experiments, reflects no more than between a 6th and a 7th part of the light that falls on 

 her. — Orig. 



f The latitude of Sirius being only 39° 33', his parallax will be a little less than \ of the whole 

 parallax — Orig. 



X We have at present no means of judging of the comparative brightness of the sun and of the 

 fixed stars, in proportion to their respective sizes, excepting from the comparison of the sun's bright- 

 VOL. XII. 3 I 



